Examples of a rotating machine include an induction machine and a synchronous machine. As a control apparatus for the induction motor, a control apparatus disclosed in, for example, Patent Literature 1 is conventionally known. Namely, as particularly shown in pages 8 to 12, FIGS. 1, 8, 9, 12, and 14, the Patent Literature 1 discloses the control apparatus for the induction motor, including a power conversion circuit that outputs an alternating-current primary voltage of a variable voltage at a variable frequency to drive the induction motor, a current detector that detects a primary current supplied from the power conversion circuit to the induction motor, a current component computing circuit that computes first and second current components from the primary current and a preset frequency command for the alternating-current primary voltage, a magnetic flux command computing circuit that computes a magnetic flux command so that an amplitude ratio of a square of the first current component to a square of the second current component is a preset predetermined value, a voltage component command computing circuit that computes a primary voltage component command from the frequency command and the magnetic flux command, and a primary voltage command computing circuit that computes a primary voltage command of the induction motor from the frequency command and the primary voltage component command, and that outputs the computed primary voltage command to the power conversion circuit.
The control apparatus for the induction motor disclosed in the Patent Literature 1 controls the current component computing circuit to compute the first current component in phase with the primary voltage component command, and the second current component shifted in phase by 90 degrees from the primary voltage component command. Therefore, the amplitude ratio of the square of the first current component to that of the second current component is equal to the preset predetermined value, and the induction motor can be thereby driven with high efficiency.
Patent Literature 2 discloses a technique for computing a slip and a slip frequency with optimum efficiency from a frequency, a voltage, and a current of an inverter, and for controlling the slip and the slip frequency with the optimum frequency to coincide with each other. Patent Literature 3 also discloses a technique for computing a slip and a slip frequency with optimum efficiency from a frequency, a voltage, and a current of an inverter, and for controlling the slip and the slip frequency with the optimum frequency to coincide with each other.    [Patent Literature 1] Japanese Patent Application Laid-Open No. 2000-175492    [Patent Literature 2] U.S. Pat. No. 5,500,581 Specification    [Patent Literature 3] U.S. Pat. No. 5,786,231 Specification
However, the conventional techniques have the following problems. The squares of the first and the second current components are applied as the power component in phase with the primary voltage component command and the second power component shifted in phase from the primary voltage component command, respectively. In addition, the current component computing circuit is controlled so that the amplitude ratio of the square of the first current component to that of the second current component is equal to the preset predetermined value. As a result, there are a problem that a current transient response when a load torque increases differs from that when the load decreases even if the load torques in both cases are at an equal operating point, and a problem that a current transient response at a light load point differs from that at a heavy load point.
If a rotation speed is rapidly changed by an impact load or the like, the slip frequency given by “(the primary angular frequency)−(rotation speed)” is also rapidly changed so as to keep a primary angular frequency constant. As a result, a current amplitude is rapidly changed, which may possibly cause overcurrent.
The Patent Literature 2 does not consider the transient property of the induction motor.
The present invention has been achieved in order to solve the conventional problems. It is an object of the present invention to provide a control apparatus for a rotating machine that can keep a current transient response constant irrespective of a change in a load torque or an operating point, and set a current amplitude within a desired range even if a rotation speed is rapidly changed by an impact load or the like.